1. Field of the Invention
This invention relates to a process for producing an anisotropic melt-phase forming polymer in which the production of undesirable degradation products is minimized; more particularly, this invention relates to a process for producing an anisotropic melt-phase forming polymer, involving separate acetylation and polymerization steps, which utilizes controlled vapor phase reflux to reduce reactant loss and maintain the stoichiometry of the process. In one embodiment, the process of this invention relates to the production of anisotropic melt-phase forming polymer having repeating units derived from (a) aromatic hydroxycarboxylic acid, (b) aromatic diol and/or aromatic hydroxy-amine and (c) aromatic dicarboxylic acid. In another embodiment, the process of this invention relates to the production of anisotropic melt-phase forming polymer having repeating units derived from substituted or unsubstituted p-hydroxybenzoic acid and hydroxynaphthoic acid.
2. Description of the Prior Art
The preparation of aromatic melt-forming polymers having repeating units derived from (a) aromatic hydroxy-carboxylic acid, (b) aromatic diol and/or aromatic hydroxy-amine and (c) aromatic diacid are described, for example, in U.S. Pat. Nos. 4,473,682; 4,746,694; 4,522,974; 4,918,154; and 4,937,310. Frequently, the preparative techniques disclosed in connection with the preparation of such polymers utilize the ester derivatives of the aromatic hydroxy carboxylic acid and aromatic diol (and/or hydroxyaromatic amine) precursors and charge these esterified derivatives, together with the aromatic diacid to a reactor as solids which are subsequently heated to initiate the polycondensation reaction.
Alternatively, it is known to charge the aromatic hydroxy-carboxylic acid, aromatic diol (and/or hydroxyaromatic amine) and aromatic diacid to a reactor together with acetic anhydride and a desired polycondensation catalyst, heat the reactor to initiate the acetylation of hydroxyl and amino groups of the reactants, remove acetic acid produced by the acetylation, raise the reactor temperature to initiate polycondensation, and allow the reaction to proceed to a desired polymer viscosity. Similarly, it is known to produce polymers having units derived from hydroxybenzoic acid and hydroxynaphthoic acid by charging the esterified derivatives of the aromatic hydroxy acid reactants to a reaction vessel, heating the reactants under vacuum to a temperature at which acetic acid is distilled from the vessel, and subsequently raising the reaction temperature while undergoing a staged pressure reduction, until a desired polymer viscosity is reached. See, for example, U.S. Pat. No. 4,161,470.
Monomer purity, material availability and economics are oftentimes factors which favor a process which starts with the diol and hydroxy-acid precursors of the ester derivatives. Maintaining stoichiometric balance oftentimes is more difficult when these precursors rather than their esterified derivatives are the starting reactants. Reaction stoichiometry can affect the properties of the polymer ultimately produced, including the molecular weight and melt viscosity thereof. The degree to which polymer properties are impacted generally depends on the extent of monomer imbalance. For certain end-use applications (e.g., fiber production), there is little tolerance for polymers produced by reactions in which there are even minor deviations from the calculated stoichiometry.
Additionally, the loss of reactants that occurs as the reaction is taken to elevated temperatures can contribute to conditions of stoichiometric imbalance. Anhydride and volatile acetate intermediate loss through distillation oftentimes contributes significantly to this imbalance. Utilizing excess amounts of certain reactants may remedy this problem to a greater or lesser degree. See, for example, U.S. Pat. No. 4,370,466 disclosing the use of excess diol. It will also be appreciated by those skilled in the art that there can be numerous competing reactions taking place during the synthesis of anisotropic melt-phase forming polymers, particularly since reaction conditions are themselves subject to change as the synthesis proceeds. In addition to the possible effect on stoichiometry in the context of acetylation/de-acetylation reactions, changes in reaction conditions can result in the formation of degradation products which, in excess amounts, can have a deleterious effect on the properties of the polymer ultimately produced.
Depending upon the particular polymer being synthesized, charging excess reactants can add significantly to process economics. Compensating for-distillation loss through the use of excess reactants may itself, in certain instances, contribute to stoichiometric imbalance. In theory, conducting both the acetylation and polycondensation portions of the reaction in a single reactor may minimize the potential for stoichiometric imbalance.
In single reactor systems, batch size and cycle time are factors which influence the rate of polymer production. In the above-described reactions, the amount of material polymerized utilizing a single reactor will generally be less than the reactor's capacity, given that the solid precursors occupy a greater volume than an acetylated melt. On a commercial scale, however, this can result in under-utilization of reactor capacity. Additionally, the use of a single reactor system does not, of itself, address the problem of distillate loss.
An object of this invention is to provide a process for producing an anisotropic melt-phase forming polymer which minimizes reactant loss and provides improved production efficiency without significantly detracting from the properties of the polymer ultimately produced.
These and other aspects of this invention are described in greater detail in the description and examples which follow.